1. Field of the Invention
The present invention relates to a novel phosphor, a process for the preparation of the same, a radiation image recording and reproducing method utilizing the same, and a radiation image storage panel employing the same. More particularly, the invention relates to a novel divalent europium activated alkaline earth metal halide phosphor.
2. Description of the Prior Art
There is well known a divalent europium activated alkaline earth metal fluorohalide phosphor (M.sup.II FX:Eu.sup.2+, in which M.sup.II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; and X is a halogen other than fluorine), as a divalent europium activated alkaline earth metal halide phosphor. For instance, Japanese Patent Publication No. 51(1976)-28591 discloses that the phosphor gives an emission (spontaneous emission) in the near ultraviolet region when exposed to a radiation such as X-rays, cathode rays or ultraviolet rays, the maximum of the emission being at the wavelength of approx. 390 nm, and the phosphor is useful for a radiographic intensifying screen employable for radiography such as X-ray photography.
Recently, it has been discovered that the divalent europium activated alkaline earth metal fluorohalide phosphor emits light in the near ultraviolet region when excited with an electromagnetic wave such as visible light or infrared rays after exposure to a radiation such as X-rays, cathode rays and ultraviolet rays, that is, the phosphor gives stimulated emission, as disclosed in Japanese Patent Provisional Publication No. 55(1980)-12143. For this reason, the phosphor has been paid much attention as a phosphor for a radiation image storage panel employable in a radiation image recording and reproducing method utilizing a stimulable phosphor.
The radiation image recording and reproducing method can be employed in place of the conventional radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt and an intensifying screen as described, for instance, in U.S. Pat. No. 4,239,968. The method involves steps of causing a stimulable phosphor to absorb a radiation having passed through an object or having radiated from an object; sequentially exciting (or scanning) the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with the conventional radiography. Accordingly, the radiation image recording and reproducing method is of great value, especially when the method is used for medical diagnosis.
As for a stimulable phosphor employable in the radiation image recording and reproducing method, a rare earth element activated alkaline earth metal fluorohalide phosphor such as the above-mentioned divalent europium activated alkaline earth metal fluorohalide phosphor is known, but almost no stimulable phosphor other than this phosphor is known.